Bipolar transistors are well known in electronics, having been invented over half a century ago. They typically have at least three terminals, a base, an emitter, and a collector.
Consider an NPN bipolar transistor T1, having a grounded emitter E, as illustrated in FIG. 1.
To turn the transistor ON, a current is applied to the base B of the transistor, in this PRIOR ART example the current is applied through resistor R1 by driving a circuit input IN1 to a predefined positive voltage level. The transistor T1 will then conduct a current from its collector C to its emitter E of up to a ratio Beta times the current flowing from base B to emitter E. Beta is a device- and operating-condition-dependent parameter subject to process variations; typically circuits are designed to operate with devices having a range of Beta ratios to ease manufacture.
It is well known that collector C voltage may be significantly less than base B voltage at some, but not all, collector currents, hence collector voltage is not limited to be more than a junction drop above the emitter voltage. Some other devices known in the art, such as insulated-gate bipolar transistors or bipolar darlington-pair 50 devices (FIG. 2), typically have higher minimum output (or collector) voltages, which may result in undesirable power losses in some circuits when conducting large currents. It is known, for example, that a darlington-pair 50 has a minimum collector 52 voltage of a base-emitter diode drop plus a collector-emitter drop, or about 0.9 volts for silicon bipolar devices. There are circuits where the lower collector-emitter voltage drop of bipolar junction transistors offers significant advantage in a system.
It is also apparent in the circuit of FIG. 1 that power is dissipated in the driving resistor R1.
It is also known that a space-charge builds up in the base-emitter junction of bipolar transistors, and to turn off a transistor this charge must be dissipated. Further, to turn the transistor on, a certain amount of space charge must be injected into the base.
In this document, reference will be made to NPN transistors, typically operating with voltages on base and collector that are positive relative to the emitter. Everything stated herein is also applicable to PNP transistors, however base and collector voltages are negative relative to the emitter for PNP transistors.
Buck converters are known in the art of power supply circuitry. These typically provide a high-efficiency voltage reduction, and current increase, between an input voltage terminal and an output terminal. In a typical personal computer, for example, a buck converter is typically used to convert an input power supply voltage of 5 or 12 volts to a local processor power supply voltage of less than 2.5 volts for use by a central processor unit (CPU) chip. Buck converters are typically operated under feedback control to maintain the output terminal near a predetermined voltage.